I just noticed that my FPV-displayed images were looking very different in colour and contrast from what Photoshop, or any other viewer (colour managed or not) were showing. I eventually tracked this down to the use of WCS in FPV. With this option selected, colours are way off.

I don't recall if/when I turned on the WCS option, or if it's on by default, but that is a non-issue. Also, I am quite content to keep on using FPV without WCS, but just out of curiosity, why does using WCS result in such an imprecise rendering? I suppose one could be tempted to say that WCS IS accurate, and every other viewer is off, but the reality is that the images displayed using WCS, are in fact far from, well, reality.

The WCS color engine (introduced in Vista and available since then) is based on the Canon Kyuanos CMS technology and is supposed to be more accurate than the previous (and default) color engine in Windows.

FPV uses the default color engine initially. The WCS engine can be selected from the program's preferences.

If your calibration system offers different options (such as Matrix vs. LUT-based profiles) you may want to play around with the settings and see what comes out.

I suppose I selected WCS because it's "supposed to be more accurate". I ran into some info that said something to the effect that ICC profiles are not compatible with WCS, but my needs are not stringent enough to get into any heavy investigation of the subject.

Not that this has much to do with the situation at hand, your comment did lead me to discover a profiling tool of which I was not aware - DispCalGUI/Argyll - which appears to be more precise than the software that came with my Spyder3 tool. This may prove to be useful.